1. Field of the Invention
The present invention relates to a fuser or a fixing device for use in an image forming apparatus such as copier and the like. More specifically, the present invention relates to a fixing device of the type wherein an image recording medium such as a sheet of paper having formed thereon a toner image is passed under pressure through a nip formed between a heat roller and a press roller of the fixing device to fix the toner image to the image recording medium, and wherein a cleaning sheet in the form of a web unwound from a sheet supply shaft is pressed against the peripheral surface of the heat roller for cleaning that surface while being wound on a sheet winding shaft driven by a motor. Furthermore, the invention relates to the cleaning sheet per se and also to an image forming apparatus equipped with the above fixing device.
2. Description of the Related Art
In a conventional fixing device of the above described type having a sheet supply shaft carrying thereon a roll of cleaning sheet in the form of a web and a sheet winding shaft for winding thereround the cleaning sheet, part of the cleaning sheet withdrawn from the supply shaft and then moving between the sheet supply and winding shafts is pressed against the heat roller of the fixing device to clean the surface of the heat roller while the sheet is being taken up by the winding shaft. In such an arrangement, it is necessary for the cleaning sheet passing between the heat and press rollers to move at a constant speed. Since the diameter of the cleaning sheet being wound on the sheet winding shaft is increased with the progress of sheet winding operation, it is necessary that the speed of the motor for driving the sheet winding shaft is slowed down in response to an increase in the wound diameter so that the speed of the cleaning sheet moving past the heat roller is maintained constant. For this purpose, a web counter for detecting the rotational speed of the motor has been provided. Since the amount of cleaning sheet wound around the winding shaft increases in proportion to the number of image recording paper sheets which have passed through the fixing device, i.e. the number of prints, the rotational speed of the motor has been controlled heretofore in response to an increase in the number of prints so that the speed at which the cleaning sheet is conveyed through the heat and press rollers is maintained constant. A manner of controlling the motor speed in response to an increasing number of prints is shown in FIG. 4.
In the above arrangement wherein the motor speed is controlled according to the curve shown in FIG. 4, the web counter must be reset to zero when a roll of cleaning sheet is replaced with a new one. If the cleaning sheet must be removed for some reason before it is used up and then replaced in the fixing device, however, information about the amount or the length of the cleaning sheet which had been used until it was removed is lost and, therefore, re-setting of the web counter cannot be made, with the result that the whole cleaning sheet must be replaced with a new roll of cleaning sheet. Additionally, the use of a cleaning sheet with a different thickness will pose an inconvenience in that the control curve as exemplified in FIG. 4 need be modified or changed to the one which suits the cleaning sheet with the new thickness.
A fixing device designed to solve the above-described drawbacks is disclosed in Japanese Patent Application KOKAI Publication No. 2000-267490. According to this proposed device, the sheet supply and winding shafts are supported in a slidable manner and there is provided a rotatable spiked wheel disposed so as to engage with the cleaning sheet adjacent to the sheet winding shaft while urging the winding shaft toward the sheet supply shaft. The rotatable spiked wheel drives the cleaning sheet to be wound round the winding shaft so that the cleaning sheet is conveyed at a constant speed. However, because the spiked wheel is rotated by a mechanical drive, which calls for a substantial space for installation thereof the proposed device is disadvantageous in terms of space factor, thereby making it difficult for the device to be constructed compact in size. If a fixing device operable according to the control curve shown in FIG. 4 is arranged in side-by-side relation to a fixing device using the above spiked wheel, there will arise an inconvenience that a cleaning sheet used only halfway in the fixing device having the spiked wheel cannot be used in the other fixing device.
The present invention has been made in light of the above-described problems and, therefore, has an object to provide a fixing device which can detect a speed of movement of a cleaning sheet with ease and with high accuracy without using any mechanical drive, thus making it possible to construct the fixing device small in size.
Further, an object of the present invention is to provide a cleaning sheet itself having on one surface thereof a pattern of markings to be detected for speed controlling.
Still another object of the invention is to provide an image forming apparatus using such fixing device and cleaning sheet.
For achieving the above objects, the present invention provides a cleaning sheet for use in a fixing device having a fixing heat roller and sheet supply or unwinding shaft and a winding shaft. In operation, the cleaning sheet unwound from the sheet supply shaft is placed in contact with the heat roller for cleaning the peripheral surface thereof while being wound on the sheet winding shaft. According to the invention, the fixing device has an optical sensor and the cleaning sheet is provided on one surface thereof with a pattern of markings which is detectable by the optical sensor in order that the speed at which the cleaning sheet is being moved is determined.
By providing such optical sensor and cleaning sheet, a speed of movement of the cleaning sheet can be directly detected with high accuracy and without calling for a complicated mechanical device.
According to the preferred embodiment of the present invention, the cleaning sheet is characterized in that the pattern of markings comprises graduations marked equidistantly along one lateral edge of the cleaning sheet.
The provision of such equidistantly-marked graduations on the cleaning sheet permits easy and accurate detection of the markings and hence of the speed of movement of the cleaning sheet.
According to the invention, part of the above graduations are marked with graduation numbers. Furthermore, the graduation numbers are provided in such a serial order that each graduation number indicates a position on the cleaning sheet which is locatable by counting from the first graduation number, e.g. zero. Still further, the graduation numbers are provided in increment of a predetermined value, e.g. 5, as 0, 5, 10, 15 and so forth.
Such graduations and their numbers on the cleaning sheet serve to provide information helpful to make sure of about what extent the cleaning sheet has been used by having reference to the graduation numbers and, therefore, such cleaning sheet is applicable easily to a fixing device which needs re-setting of a web counter depending on the amount of the web which has been already used for cleaning. The use of the optical sensor in conjunction with the cleaning sheet having the equidistantly-marked graduations can make possible easy adjustment of the sheet moving speed so that it is maintained substantially constant by controlling such that the rate of reading the graduation by the optical sensor is constant. Furthermore, the provision of the graduation numbers in increment of a predetermined value not only permits easy recognition of the used amount of the cleaning sheet, but also obviates the trouble of marking each graduation with its own number.
The present invention further provides a fixing device comprising a fixing heat roller for fixing a toner image carried on an image recording medium, a press roller for pressing the image recording medium against the heat roller, a cleaning mechanism which includes a sheet supply shaft carrying thereon a roll of the cleaning sheet in the form of a web and a sheet winding shaft for winding thereon the cleaning sheet, wherein the cleaning sheet is pressed against the heat roller for cleaning the peripheral surface thereof while being wound on the sheet winding shaft. According to the invention, the fixing device further comprises a speed detector for optically detecting the speed of movement of the cleaning sheet which is being fed to be wound on the winding shaft, and a speed control device for controlling the speed of winding of the cleaning sheet based on the sheet moving speed detected by the speed detector.
With the fixing device thus constructed, the speed of movement of the cleaning sheet can be directly detected with high accuracy without the need of a complicated mechanical device and it can be controlled with high accuracy, accordingly.
In the above fixing device, the cleaning sheet is provided on one surface thereof with a pattern of markings, and the speed detector comprises an optical sensor for optically detecting the pattern of markings to generate detection signals, and a speed calculator for calculating the speed of movement of the cleaning sheet based on the detection signals generated by the optical sensor.
By so constructing the fixing device, the speed at which the cleaning sheet is conveyed to be wound around the sheet winding shaft can be detected with ease and with high accuracy and, therefore, controlling of the speed can be accomplished with high accuracy, accordingly.
In the above fixing device, the pattern of markings comprises graduations, so that the information about the speed of movement of the cleaning sheet and the amount of the cleaning sheet which has been already used for cleaning can be obtained easily.
The fixing device according to the present invention further comprises a motor for controllably driving the sheet winding shaft, wherein the aforementioned speed control device is operable to control the speed of rotation of the motor so that the speed of movement of the cleaning sheet is maintained substantially at a predetermined value.
As result of such construction of the fixing device, the speed of movement of the cleaning sheet can be controlled with ease and with high accuracy.
The present invention also provides an image forming apparatus comprising: a fixing device including a heat roller and a press roller through which an image recording medium carrying thereon a toner image is passed for fixing said toner image to the image recording medium, said fixing device further including a sheet supply shaft carrying thereon a roll of cleaning sheet in the form of a web, a sheet winding shaft for winding thereon said cleaning sheet, and a motor for controllably driving said sheet winding shaft, said cleaning sheet being pressed against said heat roller for cleaning the peripheral surface thereof while being wound on said sheet winding shaft; said cleaning sheet being provided on one surface thereof with a pattern of markings; an optical sensor for optically detecting said pattern of markings on said cleaning sheet while said cleaning sheet is being moved to be wound on said sheet winding shaft, said optical sensor generating a detection signal when detecting each marking; and means for calculating an actual speed of movement of said cleaning sheet based on said detection signals from said optical sensor and for controlling a speed of rotation of said motor so that the calculated actual speed of movement of said cleaning sheet approaches a predetermined speed of movement.
In the image forming apparatus thus constructed, optically reading the markings on the cleaning sheet by the optical sensor makes possible direct and easy detection of the speed at which the cleaning sheet is moved to be wound on the winding shaft, which in turn permits easy and accurate controlling of the moving speed of the cleaning sheet.